Recovery of cellulose acetate from acetic acid solutions thereof



Patented SeptLIS, 1931 r 1 UNITED STATES PATENT OFFICE HANS T; CLARKE, OF ROCHESTER, NEW YORK, ASSIGNOR T EASTMAN KODAK COMPANY, OF'ROCHESTER, NEW YORK, A CORPORATION OF NEW YORK RECOVERY OF CELLUIiOSE ACETATE FROM ACETIC ACID SOLUTIONS THEREOF No Drawing.

which the cellulose acetate is formed and dis- 5 solved. One object of the invention is to provide a process which will recover cellulose acetate in a form that can be readily disintegrated, washed,.and further treated or dissolved. Another object is to provide a process in which the acetic acid will be finally obtained in such a concentrated condition that its recovery willbe facilitated and cheapened. Other objects will hereinafter appear.

When acetic acid solutions of cellulose acetate, such as the ordinary cellulose acetate reaction mixtures, are precipitated into water, the cellulose acetate separates in the form of clumps, shreds or fibers which have 2 a horny or highly resistant surface. Consequently, they are very difiicult to break up and to wash free from acid. Moreover, the acetic acid is very much diluted by the pre cipitation bath; and thus can be reconcentrated only with excessive trouble and expense.

I have found that cellulose acetate may be recovered from its solutions in acetic acid in the form of soft clumps, fibers or shreds,

which can be readily disintegrated and washed, by precipitating said solutionsinto an aqueous bath containing over 20% of acetic acid. I have also found that a thorough precipitation of the cellulose acetate takes place even if the acetic acid in the aqueous precipitation bath reaches 50%. It might have been expected that wasteful amounts of cellulose acetate would remain in solution in such a strong concentration of acetic acid, but I have found that the actual loss is negligible. Since the initial concentration of acetic acid in. the precipitation bath is at least 20%, and the final strength may be as high as 50%, said acid can economically be reconcentrated to strong or glacial form for use in acetylatingfurther batches of cellulose. In other words, by keeping the concentration of acetic acid in the precipitation bath between 20% and I obtain a full 50 recovery of the cellulose in the best condition Applicat ion filed January 6, 1928. Serial No. 245,059.

for further treatment, and at the same time obtain the acetic acid in the best strength for further concentration.

I shall now describe examples of my invention, but it will be understood that the latter is not limited to the details thus set forth, except as indicated in the appended claims. A precipita'ing bath is prepared consisting of 650 parts by weight of a 33% aqueous solution of acetic acid. Into this there is poured with vigorous stirring 275 parts of a reaction mixture in which the cellulose acetate is dissolved in acetic acid, there being in said mixture, for instance, over 38 parts of cellulose acetate and 237 parts of acetic acid, the latter being nearly glacial, say about 97% strength. The cellulose acetate comes down as a soft, readily disintegrated precipitate, without the highly objectionable resistant surface obtained in a plain aqueous precipitation bath. The precipitate is separated from the bath by any well known means, such, for instance, as straining it through a cloth filter. The final precipitate is then washed to the customary extent with plain water and is ready for further treatment or use. The bath liquor from which the precipitate was removed contains about 50% of acetic acid and contains practically no dissolved cellulosic material. It is, therefore, in excellent condition for reconcentration, say to glacial acetic acid for further acetylation of cellulose. Such reconcentration can, for example, be carried out by the process disclosed in copending applis5. cation 232,916, filed November 12, 1927, Hans T. Clarke and Donald F. Othmer, processes of removing water from aqueous acetic acid. In said process ethylene dichloride is mixed into the aqueous acetic acid solution and an azeotropic mixture of water and ethylene dichloride is distilled oil.

In another example of my invention, a bath of 650 parts of 33%acetic acid is prepared, say at room temperature. Into this, there is 95 gradually but vigorously poured and stirred 275 parts of a cellulose acetate reaction mixture containing 38 parts by weight of cellulose acetate dissolved in 237 parts by weight of 97% acetic acid. During the gradual in- 100 troduction of the reaction mixture into the precipitation bath, there is also gradually added to the latter 230 parts of water. This maintains the concentration of the bath more nearly uniform throughout the whole precipitation. The precipitate is of the same character as that described in the foregoing example, being soft and readily disintegrated and in the best condition for subsequent Washingv and other treatment.v Theclear liquor, left after the precipitate has been removed or filtered, contains about approximately 40%of acetic acid and can be veryeconomically concentrated.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is: s Y

1. In the process of recovering cellulose acetate from a solution thereof in acetic acid, the step of adding said solution to an aqueous non solvent precipitating bath containing initially at least by weight of acetic acid. 7

2 In the'process of recovering cellulose acetate from its acetic acid solution in. a reaction mixture, the step of adding saidcmixture, to an aqueous precipitation bath cone taining initiallyat' least 20% by weight of acetic acid, said bathhaving substantially a non-solvent action on the cellulose acetate, the total amount of water in said bath throughout the precipitation being at least equal to the weight of acetic acid dissolved therein.

3-. In the process of recovering cellulose acetate from its acetic acid solution in a reaction mixture, vigorously stirring said mixture into an aqueous precipitation bathcontaining initially at least 20% by weight of acetic acid and the water in said bath being not'under throughout the precipitation,

separating" the precipitate from said bath, and washing the same.

4,-In the process of recovering cellulose acetate from a solution thereof in acetic acid,

the step which comprises adding said solution to an aqueous nonsolvent precipitating bath-containing initially at least 20% by Weight of acetic acid, the said bath being in an amount sufiicient to effect precipitation of the cellulose acetate.

the step which comprises adding said solution to an aqueous precipitating bath con taining initially approximately 33% by weight of acetic acid and maintaining approximately that concentration of acetic acid in the precipitating bath throughout the precipitation. I

8. In the process, of recovering cellulose acetate from a solution thereof in acetic acid, the step of vigorously stirring said mixture into an aqueous precipitation bath containing from approximately 33% to about 4. 0% by weightof acetic acid and maintaining approximately that concentration of acetic acid in the precipitating bath throughout the precipitation.

9. In the process of recovering cellulose acetate from a solution thereof inacetic acid, the stop of adding said mixture with vigorous agitation to an aqueous precipitation bath containing; initially from, approximately 20 to 50% by weight of acetic acid and maintaining approximately that range of concentration of acetic acid in the precipitating bath throughout the precipitation.

10. In the process of-recovering cellulose acetate from a solution thereof in acetic '5.- In. the process of recovering cellulose acetate from a solution thereof in acetic acid, the step which comprises'adding said solution to an aqueous precipitating bath containing approximately 33% by Weight of acetic acid.

6.. In the process of recovering cellulose acetate from a solution thereof in acetic acid, the step of adding said mixture to an aqueous precipitation bath containing from approximately 33% to about 40% by weight of acetic acid. r

7 Inthe process of recovering cellulose acetate from a solution thereof inacetic acid, 

